Grindelwald had apparated into the village of Hogsmeade, and had the full intention of going up to the school and finding at least one of the Potter children. Who knew that it would be his luck that he'd come across one coming towards him... at least he was assuming it was James Potter. The older boy with him referred to him as James anyway. It was a good guess.
They hadn't seen him yet, but apparently they were already on their guard. "No surprise attacks then..." he muttered pulling his wand out and held it at the ready before he approached the two boys. "You would be James Potter, yes boy?" he asked looking to the younger one. It didn't matter who the other one was. From what Gellert had been able to figure out through observing the journal conversations, James was the oldest. The other was probably just a friend of some sort. Easily dispensable.